ABSTRACT [edit]: Falcarius utahensis, from the lower Yellow Cat Member of the Cedar Mountain Formation, represents the most complete and morphologically primitive therizinosaur yet discovered. This work completes the description of skeletal material prepared from the Crystal Geyser Quarry subsequent to the first five years of excavation.
Results of this study reveal a significant degree of morphological disparity between Falcarius utahensis and the evidently coeval primitive therizinosaur Beipiaosaurus inexpectus from the Yixian Formation, People's Republic of China and help characterize morphological transformations occurring in the therizinosaur lineage.
Falcarius documents that marked heterodonty – characterized by elongate, incisiform rostral teeth – is present in basal therizinosaurs and oviraptorosaurs (i.e. Incisivosaurus gauthieri, Protarchaeopteryx robusta), and either represents a synapomorphy or symplesiomorphy for these groups or an early phase in the convergent progression toward rostral endentulism. Nonetheless, heterodonty suggests that diet was a primary factor in the early evolution of both clades.
